Significance HIV is a sexually transmitted disease and a vaccine capable of preventing sexual transmission of HIV should elicit mucosal immune responses in the genital tract. Objective To determine if the transient viremia and immune response that occurs after intravaginal (IVAG) inoculation SHIV 89.6, could confer protection from IVAG SIV challenge, the SHIV 89.6 infected/exposed animals were challenged intravaginally with SIVmac239. Results Both control animals and one animal that was exposed IVAG to SHIV (but never became viremic) became viremic after SIV239 challenge. Two of the animals that been infected with SHIV89.6 became viremic after challenge with SIV but the virus loads were very low compared to the control animals. Three of the animals that been infected with SHIV89.6 completely resisted challenge with SIV239. These 3 animals all had SIV-specific cytotoxic T cell responses in the peripheral blood at the time of challenge. This is the best protection from vaginal challenge ever seen in the SIV system. Detailed studies of the immune responses in these animals demonstrated that the protection occurs despite the absence of SIV-specific IgA in vaginal secretions. The protection apparently did not require the presence of immune responses to highly variable regions of the viral envelope. The protection occurred only in animals that had been infected with the SHIV for more than 28 weeks before the SIV challenge. Future Directions We are continuing to monitor the animals to document differences in clinical outcome. We are designing studies to identify the protective immune responses in the SHIV-immunized animals. We will then develop vaccines which can elicit similar immune responses. KEYWORDS vaccine protection, mucosal immunity, SIV vaginal transmission